When lazily loading code, users need to be able to get hold of the
NgModuleFactory. For SystemJS environments, the SystemJS registry serves
this purpose. However other environments, such as modules compiled with
Closure compiler, do not expose exports object or a path based registry.
For these environments, `@NgModule` objects can include an identifier, and
the loading code can then pass `loadModule(id).then(() =>
getNgModule(id))` to the router.
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